The facilitation of the removal and remounting of dental casts upon a dental articulator in a standardized and reproducible manner is important and necessary to those engaged in the fabrication of dental prostheses. The ability to remount a dental cast which has been removed from the articulator to its original position is especially useful in situations where it is desired to use a specific articulator for several pairs of dental casts at the same time.
With respect to the prior art, dental articulators are a common and necessary apparatus utilized in the fabrication of dental prostheses and are well-known in the art. In order to construct such a prosthesis, the dentist takes impressions of the patient's maxillary and mandibular arches. Theses arches may or may not still include some of the patient's natural teeth. The impressions provide negative imprints of the arches and serve as the molds into which the raw material for forming positive dental casts are poured. These positive casts then serve as a template upon which the prosthesis may be constructed.
The dental casts are then mounted upon a device known as a dental articulator in order to facilitate construction of the prosthesis. This device allows the maxillary (upper) and mandibular (lower) casts to be maintained in the same anatomical relationship as in the mouth of the patient while the dentist fabricates the appliance. This is true in all cases, even those where only one arch is to receive a prosthesis, since the prosthesis must also conform with the related surfaces on the other arch in the patient's mouth.
A further reason for mounting the positive dental casts on an articulator is to permit the arrangement of any false teeth into a proper position for occlusion. In the case of partial dentures, the denture teeth must correctly occlude with the remaining natural teeth. With full dentures, greater tolerances are permitted in order to improve both function and aesthetics.
To obtain the correct occlusion, the dentist must account for not only the vertical bite but must also allow for a degree of lateral movement as well as anterior and posterior movement of the lower jaw. Many dental articulators are constructed in a manner allowing the dentist to simulate these movements to a great degree, thus improving the fit of the prosthesis.
The most common technique for attaching the dental casts in the articulator has been to mount them on a support plate with plaster, which is usually a gypsum--based material. In the event that the cast must be removed, the plaster "joint" must be broken, and the cast must later be remounted. It is difficult to precisely remount the cast in its previous orientation. Thus, much of the orientation and alignment procedure would have to be repeated.
A further disadvantage of this technique is that it is relatively expensive, dusty and time consuming to mount the dental casts with plaster. The powdered plaster must first be thoroughly mixed with a liquid such as water and the plaster must then be allowed time in which to set. This process is subject to error because, once the plaster has set, the orientation of the casts may not be changed without destroying the bond, as mentioned above.
Most dental articulators built to date utilize the plaster mounting techniques described above. Several experimental versions have been constructed using mechanical mounting devices such as claws or other clamping devices to supplant the use of plaster as a mounting material. In some cases, dental casts have been provided with grooves for engagement with projections located upon the articulator to facilitate removal and remounting, but there is no direct evidence that this arrangement materially assists the dentist by instilling a greater degree of reproducibility into the remounting process. For example, U.S. Pat. No. 4,169,314 discloses a dental articulator for mounting dental casts by means of a screw engaging a threaded aperture embedded into the base of the cast. Such fixed location points are not disturbed by the retaining means of the articulator and therefore permit a more reproducible reorientation of the casts during any subsequent remounting.
U.S. Pat. No. 4,371,338 describes a dental articulator having protruding locating pins for entering a fixed, unthreaded aperture in the base of both dental casts. These apertures are easier and faster to form, by means of a dental lathe, than the threaded apertures of the type described in U.S. Pat. No. 4,196,314. One aperture is preferably a round aperture in the central portion of the dental cast and the second aperture is preferably a radial slot. Each aperture receives a locating pin which protrudes from the articulator. The central pin prevents all movement of the cast except rotational movement while the pin engaging the radial aperture prevents even the rotational movement of the cast.
With the exception of articulators of the type described above, prior art articulators which eschew the use of plaster favor mechanical mounting devices. Such mechanical devices perform their intended function by acting as a clamp around the periphery of the dental casts. A serious drawback to the use of these devices, however, is their inability to reproducibly reorient a dental cast which has been removed from the articulator when remounting it later for further work. As the plaster of which the casts are made is mechanically abraded by the clamps, the dental cast tends to wobble or shift in position. Reproducibility is an absolute requirement for the fabrication of dental prostheses except in the case of the more simple crown and bridge work.
The applicants have discovered a fixture and a method which will enhance the reproducibility of the orientation which may be attained upon removing and then remounting dental casts upon a prior art dental articulator which previously utilized the plaster mounting technique. The fixture is capable of engaging and positioning a paired dental cast in a predetermined orientation to facilitate the construction of a dental prosthesis and then re-positioning the cast at the exact same position when it is placed on a dental articulator for further work.